Air brakes have been almost universally accepted for use in articulated vehicles of the tractor and semitrailer type. This acceptance has come about because articulated vehicles require means on the tractor which can be used for applying and controlling the semi-trailer brakes. Because of technological advantages such as flexible hoses and quick-connect and disconnect air hose couplers, high pressure air has become virtually the only accepted transmission medium for this purpose.
Air brakes of the drum and shoe type have been satisfactorily used on commercial highway and off-road vehicles for many years. The arcuate shoes of drum type brakes tend to wrap into the brake drums in a stop in the forward direction. This wrapping action causes magnification of the braking force called energization. Brake torque imbalance between the two front wheels, caused by variation in the brake lining friction, is magnified by this energization. Brake torques imbalance could cause steering pulls.
The modern trend to higher stopping rates for trucks, coupled with the requirement for straight-line stopping, now threatens to exceed the ability of the drum brake. Disc brakes, both hydraulically operated and vacuum-assisted hydraulically operated, being nonenergizing, have found increasing use in passenger automotive applications where higher braking performance, better straight-line stopping ability and reduction in brake fading was desired. U.S. Pat. Nos. 3,536,166, 3,768,604 and 3,835,962 in the name of E. J. Falk teach hydraulically operated disc brakes suitable for automotive applications. Similar use of disc brakes has not been made on articulated highway vehicles.
Until the present invention, practical brake components have limited the use of truck tractor disc brakes to actuation with air-applied hydraulic actuators. This limitation resulted from the need for force multiplication from the approximately 100 psig commonly available from the truck air supply to the approximately 45,000 pounds of normal force at the disc brake caliper in a large truck. Although the equivalent of such force multiplication was readily obtained using hydraulic cylinders, the hybrid air/hydraulic disc brake was complex and costly because it required both hydraulic and air actuators.